


A Thin Line

by echoflowertea



Series: Open Requests [19]
Category: Undertale (Video Game)
Genre: Alternate Universe - Underfell, F/M, Underfell Sans
Language: English
Status: Completed
Published: 2016-08-27
Updated: 2016-08-27
Packaged: 2018-08-11 10:04:06
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,714
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/7886881
Author URL: https://archiveofourown.org/users/echoflowertea/pseuds/echoflowertea
Summary: <blockquote class="userstuff">
              <p>You really didn’t think you and Sans could see eye-to-eye on anything. Being wrong never felt so good.</p>
            </blockquote>





	A Thin Line

**Author's Note:**

> **prompt** : If you are still taking requests I need some tsundere UF!Sans, maybe with a sick/injured Reader? A situation where beating or intimidating won't fix the problem

Sans had two things he hated:

  1. Pretty much every fucking monster Underground.
  2. Weaklings.



The two were not always mutually exclusive. 

He was constantly harassed by nearly everyone. His brother’s position in the Royal Guard and his cutthroat attitude earned him a lot of enemies. He learned how to deal with the pervasive hatred from other people. His life was plagued with memories upon memories of his brother’s own verbal and emotional abuse coupled with complete isolation from anyone else.

Then you came along.

He grunted and shifted in his post.

You fell into category…well, he wanted to say two, but that wasn’t true. You were weak in the sense that you had no backbone. You weren’t like everyone else. You didn’t break other people for fun. You never climbed to the top and found your spot in the food chain.

But you didn’t have to, because no monster could touch you. All of their magic went through you like it was _nothing_. All of their malicious intent passed on through with just a damning whisper to keep it company. And you never struck back. You never used it to your advantage to hurt them. You never sat there and twisted everyone to listen to your every whim and demand.

It was fucking madness. If _he_ had that power…if he wasn’t cursed with _one-fucking-hp,_ he would’ve been tearing down more monsters left and right. Cutting through their bodies without having to rely on his brother’s begrudging protection. And he would revel in it all.

So you were an outlier. You were strong, but…weak as hell. You never lifted a finger to solidify your place in the kingdom. But you could’ve destroyed him no problem if you put so much as an ounce of effort. He wasn’t sure if he found that terrifying, infuriating, or fucking _wonderful_. That someone so goddamn powerful that could crush him chose to have him stick around instead, and never under the guise of thin loyalties or familial obligations.

Well. Whatever. He didn’t _need_ you. He didn’t _need_ your weird sympathy or whatever the fuck it was that made you spare his life. That made you keep him at arm’s length at nearly all times with a cautious smile on your face.

Were all humans this weird? He flicked a hard piece of snow from the sentry station and watched it go flying. If they ever got to the surface, it would’ve been hell if they were all like you. Completely immune to their magic and so willing to forgive and befriend. It made him want to destroy something.

He roused himself up and stuck his hands in his pockets. He might as well take a quick look around and see if any twerps had fallen down here. It’d been a while, and the last kid took quite a few hits before they died. He’d heard about them making it all the way to Hotland before their SOUL was snatched up.

You made him promise to call you if he ever found someone lying face down in the snow. Pft. He would rather die than let you have all the fun. What was the point in saving kids, anyway? If they couldn’t withstand the blows of lesser monsters in the forest, then they really needed to be put down to save them from their own misery.

That was how he saw it. Before. Before…you pretty much sat him down and went off about how he was a fucking asshole.

“Ah! That’s COLD!”

“Well you’re supposed to DODGE it, dummy!”

Sans paused at the sound of your voice.

He turned the corner and watched you flop down onto the snow. You stared up at the sky with the flakes settling on your lashes. He felt something tighten in his neck and tore his eyes away, flustered. What the _hell_ was going on with his SOUL? It felt like it was trying to claw its way out of his chest. He wrapped himself tighter in his jacket and listened instead, hoping that your lilting voice would be a little less troublesome than the sight of you splayed on the ground.

“Damn it! You’re pretty good at this.”

“Of course I am! I’m gonna be part of the Royal Guard, remember?!”

Ah, it was the little brat down the street. The one that Undyne had beaten down at least ten times. They still followed her around spouting that they would make her proud, but she hadn’t given them the time of day.

“Is that really what you want?”

What kind of…Sans scowled at the rest of the scenery in front of him.

“Yeah! I’m gonna kill humans and bring SOULS to the king and everyone is gonna love me!”

That sounded kind of familiar. Sans remembered that his brother had mentioned something similar when he was younger. He’d always done it for recognition, but that power had gone to his head. Now it was all he ever saw when he looked at himself in the mirror – a pawn of Asgore, someone that would crush everyone in their path if given the chance for a taste of more power.

“Ugh. And here I thought we were getting along.” Slap. Sputter. “That’s what you get for letting your guard down.”

“Grrr! I’ll show you!”

Yeah, that was enough of that.

Sans came out through the trees, trying to look as intimidating as possible. Anything to break up the happy-go-lucky mood. He was so tired of hearing laughter torn from your throat, and it wasn’t even his doing. What did the brat do to deserve it, anyway? You reserved it so much that he’d only heard it a handful of times, so that was infuriating in itself.

Monster Kid froze. Their tail stiffened and they got into stance, straightening their back and eyes faraway. They were trying to suck up to him, and Sans wasn’t having it.

“heya. whaddya guys doin’? just, uh…havin’ a little fun?”

“N-No sir!”

“seems like you were. you wouldn’t lie ta me, wouldja, kid?”

You sighed. “Stop trying to intimidate him. MK, you seriously don’t have to worry. Nothing’s gonna happen to you while I’m here.”

“izzat so?” Sans scowled. He really hated it when you called him on his bluff. “’n why’s that?”

“Because. We’re friends.”

Ouch. His jaw went slack and he sputtered out something. He _stammered_. In front of the kid and everything! Sweat beaded at his skull and he glanced from the left to right, embarrassment settling on his shoulders. His entire world went _red_. As red as the lining of his jacket. As his magic. As his fucking _eyes_. The same color that crept up his collarbones that he shoved away because THAT was not a THING and he was going to PROVE IT.

“friends? you? n’ me? don’t make me fuckin’ laugh.” He grinned at you maliciously, the gold tooth glinting in the setting sun. “yer just a mistake.”

You regarded him with something between rage and dejection. “Wha…”

“you heard me. don’t know what kinda human you are, but you fit in just fine with the rest of us. maybe that says somethin’ about ya.”

He was having fun, now. You were speechless.

“what, got nothin’ to say? i hit a nerve?”

MK glanced at you. Then Sans. Torn on who to believe and who to suck up to. Sans liked this. He liked riling you up. It gave him life. It made him feel like he still had power. He hated that he couldn’t get to you as easily, but in instances like this, with just the right dig…he was able to shape you to do exactly what he wanted.

You bit your lower lip and dropped the ball of snow you had in your hand. Walking past him and through the trees toward Snowdin.

Sans watched you go. His initial giddiness faded, the adrenaline ebbing away as the bitter words left a sour taste in his mouth. Why the hell did he feel bad about this? Sure, he knew that you were more sensitive than most monsters. It was why he liked messing with you. But this conversation was different, and the look of absolute heartbreak on your face was enough for him to feel something akin to regret.

He went to look back at MK, but the kid had turned tail and ran.

Heh.

He was used to being alone, anyway.

Sans shuffled along and looked through the forest for anyone he could take his frustrations out on. Okay, so maybe he’d gone a little overboard. So what? You needed to get a thicker skin. Monsters said much crueler things to you since you ended up down here. And while they’d mostly stopped because they were afraid of what you were capable of, it only meant that you needed to be kept on your toes now and then. The world was cruel. You couldn’t coast through life thinking things would just bend over backwards for you.

Maybe he shouldn’t have…

No. NO. He was stronger than this. And the squishy feeling in his body was something he wasn’t going to address. Ever.

“HELP!”

Good. He needed something to do.

He whistled and grinned, heading toward the cries and pleas near the riverbank. An easy target. Anything to make his day. He said he wouldn’t crush the life out of so many people any more, but what was life without some torture once in a while?

“HEEEEEELLLLPPPP!”

Wait a second. That sounded like…

He moved a little faster. His pace picking up as he ignored the dull pang of unsteadiness that nestled in his chest. It couldn’t be.

“SOMEONE! PLEASE!!!”

That did it. Sans broke into a run. If the kid wasn’t in trouble, he was gonna fucking get it.

He came to the water’s edge. MK had their legs stretched out to the water, toes curling against something that threatened to sweep away with the current.

Your body.

Something shattered inside of him in that moment. Seeing you float along, face down in the water, with a steady trickle of blood pouring from your head was enough to make him nauseous. He shoved away all thoughts of his reputation before getting on his hands and knees, stretching far out to snag the piece of cloth that the kid so desperately hung onto.

MK helped him haul you out of the water. You were icy to the touch. Motionless.

He felt like he was going to be sick.

“what…did you do…?”

The kid stared at him with wide, glittering eyes. “I…I…”

“get. the fuck. outta here.”

“But I—”

The giant skulls materialized before Sans knew what he was doing. His magic crackled in the crisp winter air, static striking out at the kid and their every move. His left eye bled with electrical discharge as he pictured a gruesome death for them. It would be so quick. So _easy_.

But even unconscious, you told him he knew better.

His attack dissipated once the kid was out of sight. Sans scooped you up in his arms and grabbed hold of your cheeks, pushing them together and inspecting your face.

He’d never been this close before. Never even…touched you. But this wasn’t the time for that. He could ruminate on it later when he wasn’t trying to save your damn life.

You were breathing, just barely. The gentle rise and fall of your chest was a sign of that. He wasn’t sure how you managed to stay alive, bobbing up and down in the water like that, but maybe you’d just recently flipped over before he showed up. Whatever the case, he was glad to see that he still had a chance to correct things.

The blood stained his bones, stark against his bleached hands and wrists. It was steady enough to worry him. He’d seen you cut yourself before, and a quick swipe of your tongue and the wound seemed to close like magic. But this? The gash on the back of your head was deep enough that it looked like it wouldn’t stop without any pressure.

He shed his jacket off and covered you just long enough for him to take a brief shortcut back to his house.

The welcome warmth of the place gave him anxiety. He wasn’t even sure if Papyrus was home or not. If he was, his brother would never let him hear the end of it for bringing you in here and patching up your wounds.

Ah.

Sans stared down at your helpless form. At the center of your chest.

A SOUL was a SOUL.

That was what everyone said.

But the more he thought about reaching forward and taking it, the more nauseous he became. He backed off just long enough to scramble backwards and try to recollect himself.

What the hell was he supposed to do?

He didn’t know healing magic. No one did. It was a myth…right?

He wasn’t going to call somebody. He didn’t have anyone he could trust. Alphys conducted sick fucking experiments on the people she snatched up, and he wasn’t going to subject you to her poking and prodding. If anyone was doing that, it would be him – because he knew that you had limits. His brother? Fat fuckin’ chance. The guy would find some way to kill you. Undyne would’ve seen this as a perfect opportunity to torture for more information, and who else was he going to rope in for this?

“Mmph…”

Oh, shit. _Shit_. You were awake. On your own. He was dumbfounded at your resilience, but quietly relieved that you hadn’t given up just yet.

“S-Sans…” You reached up and grabbed at the collar of his shirt. He glanced down and saw the flesh along your hand pucker and ripple. You were…shivering.

“what…the hell is the matter with ya?” He grabbed your shoulders and pinned you to the ground, furious. “what the hell were you doin’ out there? in the water? you _trying_ to get yerself killed? huh?”

He wanted to scream even more. To cause a scene. Because the stuttering of his SOUL was about to drive him in-fucking-sane.

Instead of fighting back, you tugged on his sleeve and pulled him down on top of you.

Shit. You were freezing. But you were warm, too. He felt you nestle into the junction between his neck and shoulder, your lips resting against the bones of his spine. That was – that was new. He froze up once he felt the gentle puffs of air caress his body, your quivering lips trying so hard to form his name as you called out to him. It didn’t help that you refused to let go of him, no matter how much he shifted or testing your grip on his collar.

“hey…get off me. y’hear me?”

You didn’t respond. You merely brought him in closer. He swallowed hard and realized that he had you caged underneath him. You were so vulnerable and…when he pulled back, the way that your eyes glittered…

“Sorry.”

The apology was…not what he expected. He was going to snap. He wanted to strangle you. To ask you why the _fuck_ you thought it was a good idea to subject yourself to the cold, harsh waters near their icy little shithole. Why the kid was the one that had you in their grip, why you didn’t even bother going to him for help before it got too dangerous!

“MK was climbing that little rock formation and slipped. They fell in and can’t swim.”

Your speech was slurred. Your eyes were drooping.

“wake up. you’re not done explaining it yet.”

You blinked at him blearily, as if trying to make sense of where you were. Even who you were. “I think…I’m going to sleep…”

“ _no_. yer stayin’ awake. with me. now tell me the rest of what happened. _now._ ”

You struggled to form words, let alone sentences. It was starting to piss him off. “When to grab ‘im n’ they…tugged tooooooo hard. Wanna’m…catch before…started to pull…”

He was going to lose his mind. “wake. up.”

“Saaaaaans, please!” You crushed him to your chest. “S’okay…”

He tore himself away from you and ignored the pounding in his head and chest. How he was sweating bullets and covered in a thin sheen of it, tongue thick in his mouth and clenching it between his teeth so he wouldn’t say something he’d regret.

“just…get your ass into bed. and wipe yourself off, first.”

No dice. You looked at him like you hadn’t even heard him. He grunted in annoyance and stood up for a few moments, completely lost.

It took a little bit of struggling, but he eventually tossed you in his room. This…wasn’t how he pictured you in his bed. A-as if he pictured you at all. That was just…stupid as hell. He peeled away as much as he could and cranked the heat in the house up, wanting to make sure you weren’t going to freeze to death while he made some arrangements.

He left you in the house and did what he always had a good time with: threatening people.

He needed some basic health items. Food and drink that brought better monsters from the brink of death. The solid, home-brewed stuff that was sure to give you the kick you needed to wake the hell up.

“10 G.”

“I’m not selling it for that cheap, go somewhere else!”

He reached over the stand and grabbed the merchant by the collar, a feral grin while his magic rumbled beneath his chest. “ya wanna run that by me again?”

He ended up getting that discount he wanted.

On the walk back home, one of the lesser monsters shoulder-checked him and sent him in a dizzy rage.

But he knew he had to get back to you. You were waiting. So he swallowed his pride and just kept going despite the confused and outraged screams from behind him.

His bag broke just outside the door. The contents all spilled into the snow. Even the echo flower extract that was sure to give you back some of your strength. The remnants of their words that you could consume to boost your own abilities. He grunted in annoyance and dropped down to gather it all up, ignoring the quiet stares of the community while they watched on.

“fuckin’ shit. fuck this! FUCK THIS!”

He wanted to leave it there. To say to hell with it. To turn heel and get back to his post. To let you slowly freeze to death inside of his house so he wouldn’t have to worry about you crushing his SOUL sometime in the near future.

Pfft.

Yeah, right. Was that what he was _really_ worried about? That you would…that you would what, reject him? And stop coming around? That he would bare something to you…a sneaking feeling that had been building from months of hanging with you…and have you just utterly destroy it for no reason? Other than the fact that he knew you thought so lowly of him?

He ignored the mocking laughter of the neighborhood while he stuffed the last of his things in the bag. He was going to give this to you whether you liked it or not.

Except that when he found you, you’d fallen asleep. Or passed out. Whatever it was, you were unconscious and he had all this shit he’d just shelled out for.

Dammit. Trust you to disobey one simple request!

He shook your shoulders. “hey, dollface.” Dollface? What the HELL was the matter with him? “never said you could take a catnap. open up.”

You reached up and touched his face. He hated how much he relished your touch, catching himself as he leaned further into it. He wasn’t sure what it was. Maybe it was the tenderness in your eyes. Maybe it was the way his gut churned when you leaned in to trace every crack and split in his skull. Maybe it was the fact that he couldn’t concentrate on anything else, the rest of the world fading away.

The moment faded as soon as your head bobbed and you looked at him sleepily.

“fuck, i-i said take this down your piehole. yer gonna need it.”

You didn’t want to take the medication and it drove him nuts that you had the gall to refuse him. After everything he’d done! Going out of his way for it, shelling out some gold, getting weird looks from people in town as they whispered on about him! He sucked it all up just to make sure you were…recovering, and for what? So you could just spit on all his efforts?

He pinched your nose shut and forced you to breathe through your mouth. As soon as your lips parted, he shoved the entire spoonful in and went straight for your neck. Stroking it so you would swallow.

There.

All he could do now was wait. And make damn sure that you weren’t going to drift off again.

He sat in the chair next to his bed, having pulled it from the desk that he never used. Leaning on it, resting against the back of the seat, gazing at you steadily.

This was…a human.

A human that he…was protecting.

Because he…

…

Sans turned his head away with an annoyed grunt.

It didn’t… _mean_ anything that he would’ve never forgiven himself if you’d drowned out there.

Or that he liked the way that you always managed to bring him a snack on his long shifts at work.

Not even how you once reached out to grab him before he stepped in a trap in the woods, drawing him close to you as he felt all of your curves from underneath your jacket pushed up against his hard bones.

It was…

Annoying.

You were…

A nuisance.

And this…

He clenched his hand into a fist. Raked his eyesockets over your lazy smile as you played with a big ball of fur that shed from the hood of his jacket.

This was…

FUCK. NO.

He stood up abruptly. You jumped at the noise and struggled to sit up. Covered up in his jacket. In _his_ jacket. Swallowed up in the same exact thing he’d held close for years, the only constant in his life that kept him hidden away from the rest of the world and the freezing cold. It was his. It belonged to him. And you wore it so well that he felt his mouth water.

“Where are you going, Sans?”

He scowled at you. What was so great? What was it that triggered this, anyway? You weren’t strong. You weren’t a monster. You were squishy. Soft. In more ways than one. You spent your days playing in the snow with kids, for fuck’s sake. You tagged along with him in town despite the rumors hanging over his head, calling him a…a _friend_ of all things.

Ouch. There it went again. The hard spike that nestled between his ribs.

What, friend? What was so bad with that word?

“need to get some air. you stay here.”

He slammed the door behind him and grabbed his backup jacket. Traversed through the town. Lingered at the spot where he found you, the heavy snowfall already covering your tracks.

He summoned his special attack and completely obliterated the rock formation next to the riverbank. It crumbled and shattered, raining down dusty remnants that scattered along his face and reminded him of what he’d done in the past.

“Sans?”

He froze. “what. are you doin’. out of bed.”

He didn’t want to hear your excuses. He couldn’t even face you; the quiver to your voice was enough to send his resolve to jelly.

“You wouldn’t tell me where you were going. I got worried.”

Why? Why did you have to do this? He glanced at you over his shoulder.

“were you…wearin’ that? the whole way here?”

You glanced down at the jacket, adjusting it so it sat around your hips more loosely. “Yeah.”

Monsters saw you. In his jacket. Wrapped up like a gift he wanted to tear apart. He put his hand over his face and dragged it downward, anger sparking in his chest.

“just get back home.”

“But I…” You trailed off. “I didn’t want to be alone. My headache cleared up and I just…I missed you.”

Strike one.

“I know you’re pissed off at me for earlier. I swear, I didn’t mean to fall into the river! MK and I were playing and then he slipped on top of the rocks. He fell into the water and I managed to grab him and throw him to shore, but the current grabbed hold of me and pulled me under. I guess I hit my head when I went down and passed out.”

Strike two.

“Thank you for saving me.”

Strike.

Fucking.

Three.

He was going to give you a piece of his mind. To tell you that he was so _done_ with your recklessness and your bleeding heart. He would demand his jacket and force you to stumble back to your own place in the freezing cold. Punish you for everything that you made him feel, including the nervousness that blossomed and coursed through him.

When he steeled himself and went to whip around to give you a piece of his mind, he hadn’t expected you to plant both hands on his shoulders and kiss him.

Oh.

_Oh._

Heh heh heh.

Wow.

He didn’t understand the appeal back then. When you read those shitty romance comics that fell down into the dump. Why anyone would want to press their mouths against each other.

But the warmth.

The softness.

And the _wetness_.

You were the one who initiated it all. Lips pressing against his bones. Cupping the back of his skull now that you had his attention. Dipping down to plant smooch after smooch along his mouth. Eyes closed because you trusted him enough not to stare. Your body warmth exchanging with his, the chilled air and bite of the dropping temperatures fading in the backdrop of your searing kisses that turned his brain and resolve to mush.

You broke away. Licked your lips. Went back for more. Hungry. _Starving_. He knew that feeling, too. And he refused to let you get away before he had his fill, snaking his arms around you to make sure you didn’t even _think_ about making a move to leave.

You retreated just far enough for him to see his own reflection in your pupils. The taste of you lingering on his tongue. His SOUL, battered and stitched together from years of survival instincts, beating along his ribcage and stretching out tired and needy tendrils toward your own.

Not yet, he chastised himself.

“U-Um. Sorry. I didn’t even…I shouldn’t…” You sucked in a deep breath and grinned crookedly. “Wait, you kissed me back.”

“no i didn’t.” He stepped backwards. Sweating.

“Yes you did! You…kissed me, too! Sans, are you saying that you…?”

He clenched his hands into fists. Feeling warmth creep up his cheekbones. “turn around and get outta here. s'cold out.”

“Don’t change the subject.” You were giddy. It was too much for him. “You like me.”

“shut. up.”

“Sans The Skeleton has a _thing_ for me.”

“what did i _just_ say?”

“You didn’t have to. I felt it. In your _kiss_.” You couldn’t stop grinning. It was pissing him off. “And you gave me your jacket! I’m never giving it back!”

“that was a loan, not a gift. hand it over.”

“No way. You want it? You’ll have to take it back yourself!”

You darted away and your melodious laughter trailed after you. Sans stood in the middle of the field, cheekbones flushed, teeth gritted and trying to stop the copious amount of perspiration that ran down his temple.

Sans had three things he hated:

  1. Pretty much every fucking monster Underground.
  2. Weaklings.
  3. A human woman who knew exactly what to do to drive him _absolutely fucking crazy_.



But he also had one thing he loved.

He just wasn’t going to admit that yet.

**Author's Note:**

>  **please comment**! it means the world to me!
> 
> want to request a fic? [**put one in my tumblr inbox**](http://echoflowertea.tumblr.com/ask)!
> 
> consider [donating](http://ko-fi.com/A258IM#_=_)!


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